This paper is a postprint (author produced version) of a paper published in Proceedings of 2014 International Symposium on Power Electronics, Electrical Drives, Automation and Motion (SPEEDAM) and is subject to IEEE copyright.

Published paper:

M. Martinović et al., "Influence of winding design on thermal dynamics of permanent magnet traction motor," Power Electronics, Electrical Drives, Automation and Motion (SPEEDAM), 2014 International Symposium on, Ischia, 2014, pp. 397-402. http://dx.doi.org/10.1109/SPEEDAM.2014.6872026

Influence of Winding Design on Thermal Dynamics of Permanent Magnet Traction Motor

Marijan Martinović, Damir Žarko, Stjepan Stipetić, Dave Staton Tino Jerčić, Marinko Kovačić, Zlatko Hanić Motor Design Ltd University of Zagreb, Faculty of Electrical Engineering and Ellesmere, Shropshire, UK Computing [email protected] Department of Electric Machines, Drives and Automation Zagreb, Croatia [email protected], [email protected], [email protected], [email protected], [email protected], [email protected]

Abstract— This paper presents summarized electromagnetic solution [3],[4]. For traction applications interior permanent and thermal design of interior permanent magnet motor for a magnet (IPM) fits naturally due to its torque-speed low-floor tram TMK 2200. Motor geometry is optimized for characteristic suitable for wide speed range in constant power maximum torque density using differential evolution algorithm. mode of operation which can be tuned as required by proper The influences of winding configuration on power losses and design of the motor. thermal transients in the motor during one driving cycle of the tram have been analyzed. The results indicate that an optimal Tariq et. al. [6] compared different IPM machine designs number of turns per slot and parallel paths can be found to yield for torque-speed profile, efficiency and demagnetization limit minimum temperatures in various parts of the motor considering at different current loading with increased machine cooling. the intermittent character of the load. Wang et. al. [7] researched the influence of PM temperature on performance of IPM traction motors. Rahman et al. [5] Keywords— electric tram, traction motor, permanent magnet defined numerous requirements that an IPM traction motor synchronous motor, electromagnetic design, losses, thermal must satisfy of which the following are in the focus of this analysis, duty cycle, optimization research: high torque density, extended speed range, effective I. INTRODUCTION cooling, safety and reliability. The induction motor (IM) is the dominant type of motor In order to meet all requirements mentioned above in the for driving modern electric vehicles on tracks, namely trams design stage, a need for combined thermal, electromagnetic and electric multiple units. Its main advantages are low cost, and mechanical analysis of the machine arises. Selection of robustness and high reliability. the appropriate type of magnets and winding insulation class are strongly temperature dependent leading to a conclusion Boosting of efficiency in all types of electric drives in that thermal analysis is of equal importance as electromagnetic industry and transport has been made one of strategic goals of analysis, while mechanical analysis provides warranty of energy policy conducted in Europe, United States (US) and safety and reliability at maximum speed. elsewhere. For instance, new efficiency standards introduced by IEC [1], [2] are targeted towards increase of efficiency of This paper studies the possibility of replacing an existing induction and other types of motors supplied from the mains induction motor driving a low floor tram TMK2200 or variable-speed drives which are sold on the European manufactured by Crotram consortium with an IPM motor of market. Similar standards (NEMA, EPACT, CSA, CEMEO the same volume, but with increased torque rating so that six etc.) exist in other parts of the world as well. The increase of induction motors can be replaced with four IPMs. efficiency can be achieved by increasing the size of existing Special emphasis is made on influence of winding design motors and/or installing laminations with lower specific on thermal dynamics of the motor. The selected number of losses. Further improvement may be achieved with redesign of turns per coil and the number of parallel paths determines the the geometry of existing motors by means of mathematical speed at which traction motor enters the field weakening optimization. regime which affects the distribution of copper and iron losses In the case of traction drives, the limited space available on in the motor as a function of tram (motor) speed. As a the bogie of a vehicle is always a restraining factor for the consequence the maximum temperature reached in the various traction motor. In order to increase efficiency and maintain parts of the motor during one driving cycle of the tram reasonable motor size, a synchronous motor with permanent (acceleration  constant speed  braking  standstill) is magnets (SMPM) with its inherently higher torque density and dependent on the winding design. The detailed results of better efficiency than induction motor is emerging as a thermal analysis are presented in the paper suggesting the best speed and motor torque shown in Figs. 2 to 4 have been choice for winding design in this particular case. calculated. The distance between two stations is 1000 m. A driving cycle consists of acceleration to the maximum speed Goss et. al. [8] proposed a design methodology for PM AC of 70 km/h followed by driving at constant maximum speed traction motor. They considered combined electromagnetic, and braking to standstill. The tram is idle for 20 seconds until thermal and mechanical performance. The problem of the the start of a new cycle. winding design (number of turns per coil) emerged also in their research. Nevertheless, the accent was put on thermally The IPM motor must provide 705 Nm of torque during constrained continuous maximum torque (and power) acceleration and 1020 Nm during braking. envelope rather than on the losses and temperature in various active parts during complex duty cycle of IPM railway traction III. MEASURED CHARACTERISTICS OF EXISTING INDUCTION motor as in this paper. MOTOR All simulations in this paper are performed using SPEED The friction and windage losses of the IPM motor and its PC-BDC+PC-FEA and Infolytica MagNet software for through ventilation characteristic are assumed to be the same electromagnetic analysis, and Motor-CAD software for as in induction motor since both motors have the same thermal analysis. The motor design process is further bearings, cooling fan and geometry of cooling ducts. improved by applying evolutionary optimization implemented The amount of friction and windage losses is measured on in MATLAB in combination with commercial SPEED PC- TMK 2200 induction motor in no-load test conducted BDC+PC-FEA software. according to IEC standard [11]. These losses shown in Fig. 5 are used for the purpose of the IPM motor design as an input II. TRACTION MOTOR REQUIREMENTS to PC-BDC+PC-FEA software. An induction motor (IM) which drives low-floor tram TMK2200 is used in this research as a starting point for IPM Since the induction motor forces air flow through axial motor design. The IM mounted on the tram bogie and on the cooling ducts via a shaft mounted fan, in order to define test bed in the Laboratory of Electric Machines at the cooling performance of newly designed machine it is required University of Zagreb is shown in Fig. 1. The main idea is to fit to measure the air velocity in the ducts. Air velocity was the IPM motor into the frame of the existing IM to permit measured using hot wire anemometer at a variety of rotor assembly on the tram without additional modifications. Along speeds in the ducts labeled according to Fig. 5. Cooling ducts with the assumption of identical dimensions of the frame, both were accessible for measurement only on the drive end of the machines are assumed to have the same shaft, bearings and motor since cooling fan is mounted on the rear making the cooling system which result in the same friction and windage openings of the ducts inaccessible. losses. Tramcar series TMK2200 is driven by six three-phase four pole squirrel cage induction motors with the following ratings for maximum load during acceleration: 85 kW, 320 V, 195 A, 477 Nm, 1700 min-1. The maximum speed is 4580 min-1. During regenerative braking the maximum developed torque is -680 Nm. The stator has form wound coils with insulation class 200. Mechanical protection is IP20 (open motor), so a) interior of the motor is exposed to outside moisture and dirt. The torque requirements for IPM motors for the same performance of the tram will be 50 % higher since four motors will be used. The stator is assumed to have random wound coils made of round enameled wire with H class insulation and maximum allowed hot-spot temperature of 180 C. During exploitation wheels of the tram are exposed to wear creating tiny iron particles that can stick to the rotor of the IPM motor due to attracting forces of the magnets. After a certain period b) of time it can fill the air gap and damage the motor. Therefore, Fig. 1. Induction motor for low-floor tram TMK2200, a) mounted on the the IPM motor needs to be built with IP55 degree of bogie, b) mounted on the test bed in the laboratory.

80 mechanical protection (totally enclosed). 1 2 Electromechanical requirements for the traction motor are 0.5 60 0 40 determined by solving kinematic equations of the tram using -0.5

the following data: mass of vehicle and passengers, maximum -1 km/hSpeed, 20 Acceleration, m/s Acceleration, speed, maximum allowed acceleration and deceleration, -1.5 0 0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 70 80 90 maximum allowed acceleration time, maximum allowed path Time, s Time, s for stopping, wheel diameter and gearbox ratio. Based on the a) b) data found in literature [9], [10] and provided by the Fig. 2. Tram acceleration (a) and speed (b) versus time during one driving manufacturer of the tram, the traction force, acceleration, tram cycle. 4000 500 vehicle in terms of time dependence of generated power losses -1 3000 and the resulting heating of the motor is computationally too 0 2000 intensive. Therefore, the motor design is performed in two -500 Torque, Nm Speed, min Speed, 1000 steps: -1000 0 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 1) Optimization of radial cross-section by maximizing the Time, s Time, s a) b) torque density within predefined constraints, Fig. 3. Required torque of IPM motor (a) and shaft speed (b) versus time 2) Selection of number of turns per coil and number of during one driving cycle. parallel paths considering calculated thermal transient of

100 1000 the motor during one driving cycle.

50 500 The optimization of the motor design is realized using

0 0 differential evolution (DE) algorithm [12], connecting SPEED

Torque, Nm -500 PC-BDC software and MATLAB via ActiveX. In order to -50 Traction force, kN -1000 obtain reliable results, PC-FEA finite element (FE) module -100 0 10 20 30 40 50 60 70 0 500 1000 1500 2000 2500 3000 3500 4000 4500 -1 was used to take into account the influence of saturation on Speed, km/h Speed, min a) b) synchronous inductances in d and q axis. Fig. 4. Traction force on the wheels (a) and required torque of IPM motor (b) Two sets of variables are defined. The first set are versus speed during one driving cycle. variables with constant values listed in TABLE II. which are Due to uneven area of cooling ducts and their not subject to optimization. The second set are variables listed circumferential distribution on the perimeter of the stator in TABLE I. which are being optimized with respect to laminations, the measured air velocity varies for different inequality constraints defined in TABLE II. and the cost ducts. Hence, air velocity versus rotor speed is modeled as a function (torque density). The variables subject to red regression line shown in Fig. 5b. This characteristic is optimization have been normalized in order to minimize the used as an input for thermal modeling of the IPM motor. occurrence of unfeasible motor geometries that would frequently emerge otherwise. Another input to thermal model are power losses in the winding, core and permanent magnets of the motor, the The selected current density and the limit of linear current amount of which depends on the duty cycle and required density equal the values used in the induction motor at torque that traction motor must develop at a certain speed. maximum developed torque during acceleration of the tram. With known driving conditions and tram specifications [10], Since IPM motor is totally enclosed, in spite of the fact that its the duty cycle of the tram can be determined and translated rotor losses are much smaller than rotor losses in the IM, it is into the traction motor duty cycle in the form of required shaft expected that temperature of the rotor and permanent magnets torque and speed as a function of time as previously shown in will be high due to heat transfer from the stator and the fact Fig. 3. that only stator surface is forced ventilated. Therefore, the selected NdFeB magnet type is N38EH for maximum

W 2000 30 25 temperature of 200 C. 1500 20 The number of slots and poles is selected to be 36/8 which 1000 15 10 yields a two-layer fractional slot winding with distributed 500 in velocity Air 5 cooling ducts,cooling m/s overlapping coils and with good trade-off between inherent 0 0 0 500 1000 1500 2000 2500 3000 3500 4000 4500 500 1000 1500 2000 2500 3000 3500 4000 4500 -1 Speed, min-1 capability for mitigation of torque pulsations, susceptibility to Friction and windage losses, windage Friction and Speed, min a) b) noise and possibility of using multiple parallel paths. Fig. 5. Measured friction and windage losses (a), and air velocity in cooling Optimization resulted in a motor geometry with the highest ducts (b) of induction motor. torque within given stack volume which satisfies all constraints. The obtained torque density is 27.4 kNm/m3. The radial view of the optimal lamination design and field plots from FE simulation at maximum torque during acceleration are shown in Figs. 7 and 8. Minor modifications of the shapes of permanent magnet cavities in the rotor have been made with respect to the output from PC-BDC in order to better secure the magnets in position. These modifications do not affect the electromagnetic properties of the optimal design. Optimization was carried out with one turn per coil and Fig. 6. Induction motor stator and rotor lamination with axial cooling ducts one parallel path. If current density is kept the same, the (rotor bar slots and air gap not yet punched). subsequent variations in winding design do not affect the IV. OPTIMIZED DESIGN OF IPM MOTOR amount of torque the motor will developed since torque solely depends on total ampere-turns in the slot. Design of a traction motor is more complex than design of a motor for continuous duty due to intermittent character of the load. Consideration of the entire driving cycle of the TABLE I. DEFINITION OF OPTIMIZATIONVARIABLES Term Boundaries Explanation

Ds/Dso [0.45, 0.75] Ratio of stator inner diameter (Ds) to outer diameter (Dso) dys/[(Dso-Ds)/2] [0.3, 0.7] Ratio of yoke thickness (dys) to difference between stator outer (Dso/2) and inner radius (Ds/2)

bts/s [0.3, 0.7] Ratio of tooth width (bts) to slot pitch (s) at Ds λm = dm/[(Dr-Drin)/2] [0.1, 0.3] Ratio of total cavity thickness (dm) to difference between rotor outer (Dr/2) and inner radius (Drin/2) λmd1 [0.2, 0.6] Relative share of total rotor lamination depth for outermost rotor section λmd2 [0.05, 0.4] Relative share of total rotor lamination depth for middle rotor section (between the cavities) β/β0 [0.7, 1] Angle of slanted magnets (β) relative to maximum feasible angle (β0)

λp [0.55, 0.95] Angular span of the inner rotor cavity relative to the pole pitch

TABLE II. IPM MOTOR CONSTANT GEOMETRIC PARAMETERS TABLE III. IPM MOTOR INEQUALITY CONSTRAINTS Parameter Value Parameter Symbol Constraints Number of slots 36 RMS linear current density K ≤ 51.5 kA Number of poles 8 Efficiency η ≥0.95 % Stator outer diameter, mm 320 Power factor cos φ ≥ 0,8 Stack length, mm 320 Flux density in stator tooth Bst,max ≤1,8 T Shaft diameter, mm 70 Flux density in stator yoke Bsy,max ≤1,35 Air gap, mm 1 Power at maximal speed P@max_speed(5100 rpm) ≥P@rated_speed(1700 rpm) Slot opening width, mm 2,5 Voltage at maximal speed U@max_speed(5100 rpm) ≤U@rated_speed(1700 rpm) Slot opening depth, mm 1 Coil pitch 4 V. INFLUENCE OF REWINDING ON MACHINE PERFORMANCE Slot fill factor 0,4 In order to fully utilize the available electromagnetic and RMS current density, A/mm2 6.6 reluctance torque, an optimum angle  (Fig. 9) between phase Permanent magnet type NdFeB (N38EH) current and back EMF should be maintained to achieve maximum torque per ampere (MTPA) operation up to corner speed at which maximum available voltage is reached. Above corner speed reduced torque operation is achieved by adjusting the current angle to weaken the magnet flux [13], [14]. The corner speed can be changed by altering the number of turns per coil (Nc) and the number of parallel paths (ap). This variation also affects losses in the machine generated during the driving cycle of the vehicle.

Fig. 7. Stator and rotor laminations of the final optimized design.

Fig. 9. Phasor diagram of IPM motor.

Reducing the ratio Nc/ap reduces the back EMF E which entails decrease of the total phase voltage Us at a certain speed. In that case the transition to the flux weakening regime is shifted to a higher speed. The constant power speed ratio (CPSR) of the motor is not affected by rewinding of the motor [15]. It is an inherent property of the motor determined by geometric design of its cross-section, current density and magnetic properties of laminations and permanent magnets which all affect the level of saturation and motor parameters that define the CPSR. Therefore, the CPSR obtained by optimized design will be preserved. Since total quantity of copper in the slots remains Fig. 8. Magnetic flux lines and flux density plot of the final optimized design. unchanged (slot fill factor is fixed), reducing Nc/ap increases the diameter of a conductor. In that case a reduction of Nc/ap flux weakening regime at lower speed and hence yields a proportional increase of the phase current to ensure demagnetizing (d-axis) component of MMF is higher than in the same current density, MMF and torque. This is valid in windings with lower Nc/ap resulting in higher current density. constant torque regime. Above the speed (approx. 3500 min-1) at which all three In traction applications the transition from constant torque winding configurations are in the flux weakening regime to constant power region is determined by vehicle kinematics (voltage limit of 400 V is reached) the line current is the same and is not necessarily followed by traction motor design. In (Fig. 10). This is logical since at constant power and constant other words, the starting point of flux weakening regime of the voltage the same current is required if power factor and motor defined by the value of torque and speed at which efficiency are the same. maximum available voltage from the power converter is reached usually occurs at speeds higher than the speed at In order to find the winding configuration with best which constant traction power regime begins. In this particular thermal performance (lowest temperatures), the variations of case Fig. 4 indicates that constant traction power begins at power losses in various parts of the motor are calculated as a tram speed of 28 km/h, which corresponds to motor speed of function of time during one driving cycle and used as input to -1 the thermal model. All time dependent waveforms are shown 1700 min . However, the selection of ratio Nc/ap will determine the speed at which maximum available voltage will in Figs. 12 to 15. Friction and windage losses (Fig. 15), which be reached. The motor will develop constant power starting are modeled in SPEED based on measured results, depend from 1700 min-1, but this regime will be maintained by only on rotor speed and not on winding variation. reducing the motor current and increasing the motor voltage A motor with lower Nc/ap reaches the flux-weakening up to the speed at which maximum available voltage is region at a higher speed resulting in higher motor flux and reached. In this interval the current control angle  will still be higher stator iron losses (Fig. 14a). On the contrary, rotor iron kept at a value that yields maximum torque per ampere. losses, which occur due to higher order harmonics of stator Beyond that speed the current angle will be continuously MMF, decrease for lower Nc/ap due to lower current density increased thus weakening the motor flux up to maximum tram and hence lower MMF (Fig. 14b). speed. The results of transient thermal simulations, shown in In constant torque regime up to 1700 min-1 the current Figs. 16 and 17, indicate that winding with 6 turns per coil and density, copper losses and iron losses will be the same 4 parallel paths reaches the lowest overall temperatures. This regardless of the Nc/ap ratio. However, in constant power winding is the best choice from the aspect of motor efficiency regime the rate of change of current density and flux with and thermal stress considering intermittent character of the speed will be dependent on Nc/ap. Consequently, the copper load during one driving cycle of the tram. and iron losses will also be dependent on Nc/ap as the speed 800 400 increases. It should be noted that motor needs to develop the 700 300 same torque in the entire speed range regardless of the Nc/ap 600 500 200 ratio, because torque reference is governed by traction force 5 turns per coil (voltage) 400 6 turns per coil (voltage) Torque, Nm 100 7 turns per coil (voltage) requirement of the tram. 300 5 turns per coil (current) 6 turns per coil (current)

Voltage, V, Current, A Current, V, Voltage, 7 turns per coil (current) 200 0 0 500 1000 1500 2000 2500 3000 3500 4000 0 500 1000 1500 2000 2500 3000 3500 4000 The described principle is demonstrated for three winding Speed, min-1 Speed, min-1 configurations: a) 5 turns per coil, 4 parallel paths, b) 6 turns a) b) per coil, 4 parallel paths, c) 7 turns per coil, 4 parallel paths. Fig. 10. Shaft torque (a), terminal voltage and line current (b) versus speed The second winding is equivalent to the winding with 3 turns during acceleration for all three winding configurations per coil and 2 parallel paths. 7 6 2 5 turns per coil 6 turns per coil 6 5 The developed torque, voltage, line current, current density 7 turns per coil and power losses versus speed during acceleration for all three 5 4 cases are compared in Figs. 10 and 11. Similar dependence 4 3 5 turns per coil

Sum of losses in 6 turns per coil can be obtained for regenerative braking regime as well, only copper and iron, kW 7 turns per coil Current density, A/mm

3 2 0 500 1000 1500 2000 2500 3000 3500 4000 0 500 1000 1500 2000 2500 3000 3500 4000 in this case higher voltage limit of 520 V on the motor Speed, min-1 Speed, min-1 terminals is allowed. It is important to notice that motor with 7 a) b) turns needs the lowest current in constant torque regime which Fig. 11. Current density (a) and total power losses (b) versus speed during is beneficial from the aspect of power converter design and acceleration for all three winding configurations efficiency since at lower current, conducting and switching

5 turns per coil 500 500 5 turns per coil losses are lower as well. In addition, depending on standard 6 turns per coil 6 turns per coil 7 turns per coil ratings of IGBT’s available on the market, in some cases 400 400 7 turns per coil significant cost reduction may be achieved if IGBTs will 300 300 200 200 Voltage, V Current, A Current, lower current rating may be installed. Although with further 100 100 increase of N /a the motor current in constant torque regime 0 0 c p 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 will decrease, the current density and winding losses in Time, s Time, s constant power regime will increase, inevitably leading to a) b) increase of maximum temperatures in the motor. This happens Fig. 12. Terminal voltage (a), and line current (b) versus time during one because winding configurations with higher Nc/ap enter the driving cycle.

2 5 turns per coil 5 5 turns per coil 8 6 turns per coil 6 turns per coil before final conclusion can be made on the best winding 7 turns per coil 4 7 turns per coil 6 configuration for the motor. 3 4 2 ACKNOWLEDGMENT 2 1 Copper losses,Copper kW Current density, A/mm

0 0 This research if funded by European Union under the 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 Time, s Time, s project titled Permanent Magnet Machine Technology for a) b) Boosting the Energy Efficiency in Traction and Marine Fig. 13. Current density (a) and copper losses (b) versus time during one Applications (MAGEF), Contract No. IPA2007/HR/16IPO/ driving cycle. 001-040508. The purpose of this project is to enhance the

1.2 capability of University of Zagreb for research and 6 5 turns per coil 5 turns per coil 6 turns per coil 1 6 turns per coil 5 7 turns per coil 7 turns per coil development of PM machines for applications in transport, 4 0.8 power engineering and industry. The authors also thank the 3 0.6 project associate Končar Electric Vehicles Inc., Croatia for 2 0.4 1 0.2 providing valuable data on TMK2200 tram. Rotor iron losses, kW losses, iron Rotor Stator iron losses, kW 0 0 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 Time, s Time, s REFERENCES a) b) [1] IEC 60034-30 Ed. 1: Rotating electrical machines - Part 30: Efficiency Fig. 14. Stator (a) and rotor (b) iron losses versus time during one driving classes of single speed, three-phase, cage induction motor (IE Code), cycle. International Electrotechnical Comission (IEC), 2008

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150 Temperature of magnet, °C 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 [7] A. Wang, L. Heming, C. T. Liu. "On the material and temperature Time, s Time, s impacts of interior permanent magnet machine for electric vehicle a) b) applications." Magnetics, IEEE Transactions on 44, no. 11 (2008): Fig. 16. Winding (a) and permanent magnet (b) temperature versus time 4329-4332. during one driving cycle. [8] J. Goss et. al., "The design of AC permanent magnet motors for electric

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